Chimeric proteins with functions not found in natural proteins are made by fusing diverse DNA elements and expressing the chimeric genes in suitable expression systems. However, it was not known that functional chimeric proteins could be made and expressed, wherein a foreign polypeptide was placed within a host protein. Previous work included addition at the amino or carboxyl termini of natural proteins and involved alterations in the specificity of the binding of the chimeric proteins to the cellular membrane of the target cells. Thus, there was no teaching, suggestion or description in the prior art enlightening and motivating one of ordinary skill in the art of constructing a hybrid protein so that a foreign protein could be delivered across cellular membranes into the cytosol of the host cells. A formidable problem that needed to be considered was the fact that cellular membranes pose a barrier to the translocation of foreign proteins into the cytosol of target cells where the chimeric protein is desired to be introduced and the molecular mechanisms of how some normally internalized proteins reached the cytosol of the cell were not entirely known.